The ability to view satellite images online has been around for years in the form of Microsoft's Terraserver (and also on a mapping site that I can't locate right now...I swear Mapquest let you switch back and forth between the two views, but I can't find it), so this really isn't anything new. Terraserver lets you zoom in/out, move around the map, and view other versions of the map (they have a topological version), and I know that many of the people who are so excited about Google Maps are familar with it. So why is everyone so excited about it?

Part of it is Google's involvement...they draw a crowd of attention anytime they do anything these days. But it also has a lot to do with someone I wrote about a couple of years ago: it's the user experience, stupid:

Robert Morris from IBM argued last year at Etech 2002 that -- and I'm paraphrasing from memory here -- most significant advances in software are actually advances in user experience, not in technology. Mosaic was not an advancement in technology over TBL's original browser. Blogger is a highly-specialized FTP client. IM is IRC++ (or IRC for Dummies, depending on your POV). The advantages that these applications offered people were user experience-oriented, not technology-oriented.

The satellite feature on Google is no exception. They took something that's been around for years, made it way easier to use (reposition & zoom maps without reloading, pinpoint addresses and routes onto the satellite imagery, toggle between sat and road maps, map size automatically scales to the browser window, etc.), and suddenly this old thing is much more useful and fun to play around with. Ajax is the underlying technology (which isn't new either) for many of the notable Google Maps features, but how Google used it to make a useful user experience is the real story here.